1. Field of the Invention
This invention relates to a film forming apparatus for forming deposited films on a substrate.
2. Description of the Prior Art
The following apparatus is known as a film forming apparatus, for example, a plasma CVD apparatus which can efficiently produce electrophotographic photosensitive drums.
That is, an apparatus in which a cathode electrode is constructed, use is made of a cover electrode having a pair of semicylindrical portions opposed to each other so as to form a cylindrical structure when joined together and movable back and forth relative to each other, a cylindrical substrate as an electrophotographic photosensitive drum is contained in said cover electrode in insulated relationship with the latter, reaction gas is injected into the cover electrode under a vacuum atmosphere and a high frequency power is supplied to between the cover electrode and the cylindrical substrate, whereby electrical discharge is caused between the two and films of amorphous silicon, for example, are deposited on the outer peripheral surface of the cylindrical substrate.
However, such a conventional film forming apparatus suffers from the following problems.
When the reaction gas is decomposed, films of polysilane, for example, accumulate also on the inner peripheral surface of the cover electrode. Accordingly, the films of polysilane thus deposited on the inner peripheral surface of the cover electrode must be removed each time one cycle of film formation is terminated. For that purpose, the inner side of the cover electrode must be subjected to dry etch cleaning. However, this dry etch cleaning takes a long time, e.g. about two hours and accordingly, the manufacturing efficiency of electrophotographic photosensitive drums becomes very low. Also, for this dry etch cleaning, use is made, for example, of a mixture of CF.sub.4 and oxygen, and the film of polysilane decomposed in a high frequency plasma and adhering to the inner peripheral surface or the like of the cover electrode is gasified and removed. Accordingly, in the dry etch cleaning, strongly corrosive gases are produced and therefore, materials excellent in corrosion resistance must be used for the cover electrode, etc., and this leads to a very high cost of facilities. On the other hand, with such a point taken into account, there has also been proposed an apparatus in which a readily removable cylindrical insulator for causing polysilane and the like to adhere to the inner side and preventing polysilane and the like from adhering to the cover electrode, etc. is disposed between the cover electrode and the cylindrical substrate. In such an apparatus, however, an extra member such as the cylindrical insulator intervenes between the cover electrode and the cylindrical substrate. Therefore, it may become difficult to uniformly supply reaction gas to the entire circumference of the cylindrical substrate and as a result, it may become impossible to cause films of amorphous silicon to deposit uniformly on the outer peripheral surface of the cylindrical substrate. Also, discharge becomes unstable in the vicinity of the seam of the cover electrode and therefore, the thickness and quality of the accumulated films become non-uniform. Further, the cylindrical insulator is formed of alumina, quartz or the like. This has led to a problem in the durability of the cylindrical insulator, which in turn has led to a problem that caution must be exercised in handling the cylindrical insulator.